Cellular phones and other like products often use two devices for generation of sound to be heard by a user. A transducer acts as a receiver, which in simple terms is a receiver or speaker. A separate unit, such as a buzzer, functions to provide alert signals to the user. Alert signals include ring tones when a call is being received, system signals such as to indicate a low battery, and other sounds such as tones for electronic games. These alert tones generally need to be of sufficient volume to be heard in a noisy environment. In some cases the buzzer may instead be a second speaker. Similar functions are provided in this configuration: One speaker generally acts to provide sound for a user to hear in a phone call, and the other speaker provides sound for alert functions.
With the introduction of polyphonic ring tones, where instead of just rings or buzzes the alert could be a song or portion of a song, a buzzer is less desirable because it cannot produce the tonal qualities necessary. The speaker function for polyphonic and alert functions requires a higher sound pressure level than standard receiver functionality permits. A speaker is therefore preferred for this application. Using a speaker that provides voice sound for the alert function, however, generally causes the required sound pressure level (SPL) limit to be exceeded. Numerous standards, including OSHA (Occupational Safety & Health Administration), TIA (Telecommunications Industry Association), CTIA (Cellular Telecommunications and Internet Association) standards specify a safety limit, currently 115–120 dBSPL to prevent hearing loss in normal use. This limit is below the required level of most alert functions, which is typically around 135–140 dBSPL.
Some attempts at solutions to these problems include (1) using a separate speaker for the alert tones and (2) using a single speaker to perform alert and receiver functions with a variety of mitigation techniques. When a separate speaker is used for alert tones, often the sound from the separate speaker is directed out the sides or back of the phone. This prevents those higher sound pressure level tones from being able to project from the normal receiver location. Further, use of the second speaker has implications on the cost and size of the unit.
When a single speaker is used to perform alert and receiver functions, one approach is to port the majority of the sound out the side or back of the phone to limit the SPL in the receive mode to 115–120 dBSPL. This results in a loss of privacy, however, since all conversation is then similarly ported out the side, back, or both. Additional design features may be added, such as mechanical switches, or proximity switches that activate the mechanical switches, to close off the side or back ports. Though this can limit privacy loss, it generally adds cost and complexity, and in the event that there is an alert signal generated while the side or back ports are closed, SPL levels can result that exceed the 115–120 dBSPL requirement at the receiver. Proximity switches can also be somewhat unreliable or can result in undesired reduction in alert signal volume when the phone is carried on the person, as in a case, a pocket, or other enclosure.
Another option includes software solutions where the alert tones are ramped from a lower level to a high level over a set time period to allow the user to manually move the phone away from the ear before the high level sounds are generated. The possibility still exists, however, for an individual to accidentally have the receiver to his or her ear when an alert sound is being generated.